Question

how do you know if an element is an atom or an ion?

Answer 1

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You can identify atoms when they have an electrical charge. Either + or -.
Positive ions are often called cations while negatively charged ions are anions

Answer 2

All elements have positively charged protons and negatively charged electrons. When they have the same number of each, the charges balance out and you have a neutral atom. When you have a little more electrons than protons or vice versa, there is an imbalance and the element is an ion with a charge. Having more electrons makes it a negative ion, and having more protons makes it a positive ion.

Note, however, that the only thing that changes hands is electrons. You can't have the same element while changing the protons. So Fluorine, for example, often wants to gain another electron to become F (-1 charge). If you took away a proton to get a -1 charge, you would no longer have Fluorine but its next door neighbor, Oxygen!